1. Field of the Invention
The present invention relates to a method of fabricating and/or conditioning electrically conductive and electrochemically oxidizable and/or reducible polymeric electrodes for use in a battery or electrochemical storage cell, in particular a secondary cell. A battery or electrochemical storage cell utilizing an electrode conditioned by the method of the present invention can be at least substantially free, or completely free, of metal components.
2. Description of the Related Art
Since the discovery that polymeric materials, and in particular polyacetylene, could be reversibly doped and undoped and thus employed as electrode materials for charge storage applications, much consideration and investigation have been directed towards employing polymers in a wide variety of electrical and electronic device applications, including energy storage [R. B. Kaner et al., J. Phys. Chem., 90, 5102 (1989); K. Kaneto et al. Japn. J. Appl. Phys., 22, L567 (1983)], light emitting diodes [D. Braun et al., Appl. Phys. Lett., 58, 1982 (1991); J. J. M. Halls et al. Nature, 376, 498 (1995); M. Granstrom et al., Science, 267, 1479 (1995)], sensors [J. W. Thackeray et al., J. Phys. Chem., 89, 5133 (1985); G. Fortier et al., Biosensors and Bioelectronics, 5, 473 (1990); P. N. Bartlett et al., J. Electroanal. Chem., 224, 27 (1987)], and electrochromic devices [H. Yashima et al., J. Electrochem. Soc., 134, 46 (1987); M. Gazard, Handbook of Conducting Polymers, Vol. 1, ed. (1983)].
The conductivity of neutral polymers can be dramatically increased by chemically doping the polymers in a controlled manner with electron acceptor and/or electron donor dopants. The term doping used in connection with conducting polymers refers to the partial oxidation (p-doping) or partial reduction (n-doping) of the polymer, combined with the associated transport of charge compensating dopant ions into or out of the polymer. Conducting polymers are characterized by their ability to be switched between a neutral (or insulating) state and one or more doped (conducting) state(s).
In charge storage applications, such as electrochemical secondary storage cells, electrode materials should be able to undergo multiple doping and undoping cycles with high utilization efficiency and chemical stability. In addition, the two electrode materials should have a high charge capacity and combine to exhibit a high cell voltage.
Polyacetylene, polypyrrole, polyaniline, polythienylene, and polythiophene are among the several polymers that have been investigated and drawn intense interest to date in connection with charge storage applications. For example, a polymeric storage cell with a polypyrrole (cathode) electrode and polypyrrole/polystyrene sulfonate (anode) electrode is described in U.S. Pat. No. 5,637,421 to Poehler et al., the complete disclosure of which is hereby incorporated by reference.
However, repeated doping and undoping during charging and/or discharge may cause degradation of the polymer. Many polymers, such as polyacetylene, have been plagued by poor charge/discharge cycling characteristics (i.e., reversibility) due to inferior chemical and electrochemical stability. Limited improvement in charge capacity and reversibility has been reported in connection with the p-doping of poly (3(4-fluorophenyl) thiophene), this polythiophene derivative exhibits improved charge capacity reversibility when n-doped. Further improvement was shown by varying the position and number of electrophilic substituents on a phenyl thiophene. See U.S. Pat. No. 5,733,683, Searson et al., Y. Gofer, J. G. Killian, H. Sarker, J. O. Poehler, P. L. Searson, J. Electrochem. Soc. 443, 103-115. (1998) the complete disclosures of which are fully incorporated herein by reference.
Thus, while some progress has been made in understanding conduction mechanisms, electronic structure, doping characteristics, and optical properties in conductive polymers, there remains the need to develop a method of fabricating improved polymeric electrodes for electrochemical storage cells that exhibit suitable charge capacities and reversibilities in both the n-doped and p-doped states and can be employed in commercial applications without the need for metallic components.
It is therefore an object of the present invention to provide a method of conditioning polymeric electrode materials so as to improve their charge capacities and enhance their abilities to undergo multiple doping and undoping cycles with high cycling efficiency and chemical stability.
In accordance with the principles of the present invention, this and other objects are achieved by a method of electrochemical conditioning of an electrode prepared from a polymer having a lower polymerization potential than p-doping peak. Especially suited for this invention are (1) a series of fluorophenyl thiophene polymers synthesized by devising preparatory techniques to systematically vary the number and position of the fluorine on the phenyl group; and (2) a series of different phenylene-thienyl based polymers synthesized by devising preparatory techniques to systematically vary the number and position of the fluorine on the phenyl group. These polymers are disclosed in related co-pending U.S. Patent Application No. 08/961,100 filed Oct. 30, 1997, which is fully incorporated herein by reference.
The electrochemically conditioned electrode, prepared according to the method of the present invention, exhibits an improved high electrochemical stability and charge capacity for both n-doping and p-doping, particularly in a sulfolane-based electrolyte.
A substantially metal-free cell can be constructed which employs electrodes conditioned by the method of the present invention. Such a cell could be manufactured by providing non-metallic current collectors carbon black loaded polyethylene, and supports, such as graphite current collectors and poly(tetrafluoroethylene) (TEFLON) supports.
Since cells employing electrodes manufactured by the method of the present invention can be fabricated from multiple polymer films, the cells are lightweight and flexible and do not have the safety and environmental concerns associated with conventional high performance batteries. The elimination of any metallic components or liquid phases provides a unique alternative for battery technology.
Further, since the components of such a cell are both moldable into various shapes and flexible, the cell can be incorporated into a device, such as a lining, and therefore takes up much less space in the device. This feature makes an electrochemical cell.fabricated by the method of the present invention especially adaptable for application in battery-operated automobiles and satellites, and other compact devices.
The present invention still further relates to batteries and electrolyte storage cells containing electrodes electrochemically conditioned according to the method of the present invention. The battery may be either of a single cell structure or a multi-layer cell structure, and can be practiced as a primary or secondary battery.